So far, mimeograph has been widely used as an expeditious and inexpensive printing system. According to this system, a material comprising a suitable porous backing sheet such as paper and a thermoplastic resin film layer laminated on its surface is used as a heat-sensitive stencil paper. This stencil paper is cut by a thermal head or other means, and the thermoplastic resin film layer is then heated and melted to form an imagewise perforation pattern, through which printing ink is fed to make prints on the material to be printed.
In order to improve the setting properties of stencil paper used with such a thermal setting system as mentioned above, especially, the capability of stencil paper to be perforated - hereinafter simply referred to as perforability, the choice of material and the selection of a bonding agent used for laminating the thermoplastic resin film on the porous backing material present important conditions, because this system is unique. As set forth in JP-A-58(1983)-147396 and 62(1987)-264998 specifications, thermal stencil paper products have heretofore been known in the art, which are obtained by bonding together a porous backing material and a thermoplastic resin film through an adhesive layer having a network or fine regular pattern.
When the backing material and thermoplastic resin film are laminated together with such an adhesive layer having a network pattern as set forth in JP-A-58-147396 specification into stencil paper, a perforating problem arises depending upon the amount of the adhesive applied, causing the deterioration of the resulting image quality.
In the case of stencil paper including an adhesive layer having such a specific, regular pattern as disclosed in JP-A-62-264998, it is awkward in itself to form an adhesive layer having such a regular pattern. According to the inventor's finding, even when the given pattern has been formed, there are such problems as whitening and moire depending upon how much adhesive is applied and to what extent bonding takes place, which in turn occasion various problems in making printing of high resolving power.
Thus, it is a primary object of this invention to provide a thermal stencil paper which can be well cut or perforated and makes printing of high resolving power feasible.
Incidentally, thermal stencil paper used with the above-mentioned conventional, thermal mimeograph system is formed by laminated a thermoplastic resin film layer as thin as a few .mu.m in thickness on a porous backing material, generally paper, with the application of a bonding agent. This bonding agent is typically (1) a solvent (or aqueous) type of adhesive--see, e.g. JP-P-47(1972)-1188 and 1187 publications.
Problems with the solvent type of adhesive, which is used with large amounts of solvents, are that its recovery takes much cost, difficulty is involved in maintaining a working environment, the resulting products are poor in resistance to solvent, and the kind of ink used is limited.
Problems with the aqueous type of adhesive are that the quantity of heat needed for drying is enormous, and the thermoplastic resin film shrinks or the porous backing material suffers dimensional changes due to the heat applied during drying, making stencil paper curl or wrinkle.
(b) a solventless type of curing adhesives which are used for eliminating the above-mentioned defects of the solvent type of adhesives see JP-A-61(1986)-286131, 58(1983)-153697, 62(1987)-181374 and 63(1988)-233890 specifications.
Of these adhesives, the heat curing type of adhesive requires a large amount of heat for curing, and further offers problems that the thermoplastic resin film shrinks or the porous backing material undergo dimensional changes during the production of stencil paper, making the stencil paper curl or wrinkle.
The room temperature or moisture curing type of bonding agent has a defect of curing so slowly that it takes so much time to produce stencil paper; in other words, this is inferior in the productivity of stencil paper.
The ultraviolet curing type of adhesive has again a slow curing rate. At an increased dose, so great a rise in temperature takes place due to infrared rays other than ultraviolet rays, that the thermoplastic resin film shrinks, making stencil paper curl or wrinkle.
The solventless type of adhesive has a general defect of having a viscosity too high to be applied on the thermoplastic resin film or backing material to form a thin film thereon. Particular difficulty is involved in the stable application of it on a limp, thermoplastic resin film because of its viscosity.
When the adhesive is heated to decrease its viscosity, the thermoplastic resin film deforms, rendering its coating difficult. For that reason, it has been proposed to coat the adhesive on the backing material see JP-A-61(1986)-286131 specification. In this case, however, when the span of time required for curing is increased, the backing material is so impregnated with the adhesive that any product of excellent resolving power and image quality cannot be obtained.
The curing type of adhesive is inferior in its heat fusibility after curing and, hence, causes the resulting stencil paper to become worse in terms of perforability, failing to provide any product of high resolving power and excellent image quality.
Thus, a second object of this invention is to achieve economical provision of thermal stencil paper which is free from such problems as mentioned above and so serves well.
As the thermal head of a digital type of thermal mimeographing equipment, use has so far been made of a thin type of thermal head glazed all over the surface, as illustrated in FIG. 3. In some attempts to increase the perforability of stencil paper, the thermal head has been mechanically heated, or its contact with stencil paper has been improved - see JP-A-60(1985)-147338, 60-208244 and 60-48354 specifications.
In other efforts to increase the perforability of stencil paper by making some modifications thereto, the physical properties of the associated thermoplastic resin film, i.e., the thickness, thermal shrinkage factor, crystallinity, etc. thereof have been varied - see JP-A-62(1987)-2829, JP-A-63(1988)-160883, JP-A-62-149496 and JP-A-62-282984 specifications. In the case of a film formed of a polyethylene terephthalate homopolymer in particular, the perforability is satisfied only when the film has a thickness of at most 2 .mu.m, as set forth in JP-A-60(1985)48398 specification.
The adhesive, whether it is of the solvent type or the solventless type, is applied at a coverage of 0.5 to 3 g/m.sup.2 on solid basis see JP-A-1(1989)-148591 and JP-A-62(1987)-1589 specifications.
When the thermal head used is a conventional thin type of full-glazed thermal head, such as one shown in FIG. 3, there is a problem that the film of stencil paper cannot be fully perforated corresponding to the heat emitter element of the thermal head. This is because the heat emitter portion is so concave that its contact with the film is in ill condition.
In order to provide a solution to this problem, it has been proposed to heat the platen - see JP-A-60(1985)-147338 specification or prevent heat from radiating to the platen see JP-A-60-48354 specification. However, such proposals are not so effective because it is the porous backing material of stencil paper that comes in contact with the platen, and result in increased power consumption as well.
In addition, it has been proposed to use a thick film type of thermal head including a convex heat emitter portion in combination with a thin film type of thermal head - see JP-A-60(1985)-208244 specification. This proposal is considered effective for perforability, but presents a problem that the resistance value of the thick film type of thermal head varies so largely that it is impossible to obtain perforations corresponding to the magnitude of the heat emitter element.
Turning on the other hand to the physical properties of the thermoplastic resin film of stencil paper, especially, its thickness, the thinner than 2 .mu.m the thickness, the better the perforability. However, this gives rise to a serious rise in the production cost of stencil paper, or makes the rigidity of stencil paper insufficient, only to offer a problem in connection with feeding it through a printing machine.
Further, it is effective to form the resin of a copolymer, thereby lowering the melting point of the film see JP-A-62(1987)-2829 specification. However, the copolymer degrades the heat resistance, solvent resistance, etc. of the film, so that the processability of the film drops at the time of being laminated onto the porous backing material, or the resulting stencil paper becomes poor in storage stability. The copolymer also lowers the dependence of the film's viscosity upon temperature and so causes stringing, having less influence upon the perforability than expected.
A problem with the adhesive is that the larger the coverage, the better the wear resistance of stencil paper but the lower the perforability of stencil paper. When a solvent type of adhesive is used, there is a problem that skinning takes place among fibers at the time of drying, making not only perforability but also the passage of ink worse.
It is therefore a third object of this invention to provide a thermal mimeograph paper and a printing process, with which the above-mentioned problems can be solved.
Thermal mimeograph paper used with the aforesaid conventional thermal mimeograph system is generally formed by laminating a thermoplastic resin film as thin as a few .mu.m in thickness onto the surface of a porous backing material such as paper. However, because the thermoplastic resin film layer is meltable by heating, there is a problem that the thermal head may be fused to the thermoplastic resin film layer during stencil-making, thus failing to feed stencil paper stably.
In order to avoid this, it has been proposed to form a layer of such a lubricator as silicone oil, silicone resin, a crosslinked type of silicone resin or a phosphate ester on the thermoplastic resin film layer as a thermal fusion preventing layer, thereby preventing the fusion of the thermal head thereto - for instance, see JP-P-63(1988)-233890 and JP-A-61(1986)-40196, 61-164896, 62(1987)-33690 and 62-3691 specifications.
However, problems with the silicone oil are that it is inferior in the capability to form a film; it is less wetting, but repellant, with respect to the thermoplastic resin film, thus failing to form any satisfactory film; and it may contaminate other articles. This is also true of the silicone resin. In addition, oil or scum accumulates on the thermal head, and a type of silicone resin well capable of forming a film is poor in releasability. The crosslinked type of silicone resin, because of its high heat resistance, makes the perforability of the thermoplastic resin film worse. Problems with the phosphate ester are that it is poor in the capability to form a film and causes separation of the thermal fusion preventing layer, giving rise to accumulation of oil or scum on the thermal head. Use of the phosphate ester in combination with a binder presents a similar problem in connection with peeling and scumming, because it is inferior in the compatibility with the binder.
A further problem with the conventional thermal fusion preventing layer is that its insufficient antistatic properties make the feeding of stencil paper so worse that it is likely to stick to a drum during stencil-making or printing.
It is therefore a fourth object of this invention to achieve economical provision of thermal mimeograph paper with which the above-mentioned problems can be solved, and which shows excellent performance with no accumulation of oil or scum on the thermal head even when continuously used to make stencils.